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Trivial cases of shared_ptr and weak_ptr failing

std::enable_shared_from_this; public vs privateIs there any advantage of using map over unordered_map in case of trivial keys?shared_ptr and weak_ptr differencesWhen is std::weak_ptr useful?Difference in make_shared and normal shared_ptr in C++shared_ptr, weak_ptr and circular dependenciesCleaning data after exception on class constructorHow does shared_ptr<T> detect that T derives from enable_shared_from_this<T>?Crash when destroying weak_ptr, double destruction of weak_ptr's shared_ptr_emplace objectenable shared from this crashAssigning shared_ptr to weak_ptr

.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;

10

I'm having trouble using shared_ptr and weak_ptr along with enable_shared_from_this.

When I google the symptoms of what I'm seeing, everybody suggests "you cannot use shared_from_this() when there are no shared_ptr instances owning your object.

But that's not my case.

Consider this code:

#include <memory>
#include <cassert>

class MyClass : std::enable_shared_from_this<MyClass>

public:
 void this_fails()
 
 // Doesn't even assert(), because it throws bad_weak_ptr
 assert(shared_from_this());
 
 void this_fails_too()
 
 std::weak_ptr<MyClass> weak = weak_from_this();
 std::shared_ptr<MyClass> strong = weak.lock();
 // This assert fails
 assert(strong.get());
 
;

int main()

 std::shared_ptr<MyClass> obj = std::make_shared<MyClass>();

 obj->this_fails();
 obj->this_fails_too();

Both methods in MyClass crash the program. I must be missing something obvious - what is it?

c++ shared-ptr weak-ptr private-inheritance enable-shared-from-this

share|improve this question

edited 3 hours ago

curiousguy

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asked 8 hours ago

LubosDLubosD

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add a comment | 

10

I'm having trouble using shared_ptr and weak_ptr along with enable_shared_from_this.

When I google the symptoms of what I'm seeing, everybody suggests "you cannot use shared_from_this() when there are no shared_ptr instances owning your object.

But that's not my case.

Consider this code:

#include <memory>
#include <cassert>

class MyClass : std::enable_shared_from_this<MyClass>

public:
 void this_fails()
 
 // Doesn't even assert(), because it throws bad_weak_ptr
 assert(shared_from_this());
 
 void this_fails_too()
 
 std::weak_ptr<MyClass> weak = weak_from_this();
 std::shared_ptr<MyClass> strong = weak.lock();
 // This assert fails
 assert(strong.get());
 
;

int main()

 std::shared_ptr<MyClass> obj = std::make_shared<MyClass>();

 obj->this_fails();
 obj->this_fails_too();

Both methods in MyClass crash the program. I must be missing something obvious - what is it?

c++ shared-ptr weak-ptr private-inheritance enable-shared-from-this

share|improve this question

edited 3 hours ago

curiousguy

5,02922 gold badges3030 silver badges4646 bronze badges

asked 8 hours ago

LubosDLubosD

32311 silver badge1212 bronze badges

add a comment | 

I'm having trouble using shared_ptr and weak_ptr along with enable_shared_from_this.

When I google the symptoms of what I'm seeing, everybody suggests "you cannot use shared_from_this() when there are no shared_ptr instances owning your object.

But that's not my case.

Consider this code:

#include <memory>
#include <cassert>

class MyClass : std::enable_shared_from_this<MyClass>

public:
 void this_fails()
 
 // Doesn't even assert(), because it throws bad_weak_ptr
 assert(shared_from_this());
 
 void this_fails_too()
 
 std::weak_ptr<MyClass> weak = weak_from_this();
 std::shared_ptr<MyClass> strong = weak.lock();
 // This assert fails
 assert(strong.get());
 
;

int main()

 std::shared_ptr<MyClass> obj = std::make_shared<MyClass>();

 obj->this_fails();
 obj->this_fails_too();

Both methods in MyClass crash the program. I must be missing something obvious - what is it?

c++ shared-ptr weak-ptr private-inheritance enable-shared-from-this

share|improve this question

edited 3 hours ago

curiousguy

5,02922 gold badges3030 silver badges4646 bronze badges

asked 8 hours ago

LubosDLubosD

32311 silver badge1212 bronze badges

#include <memory>
#include <cassert>

class MyClass : std::enable_shared_from_this<MyClass>

public:
 void this_fails()
 
 // Doesn't even assert(), because it throws bad_weak_ptr
 assert(shared_from_this());
 
 void this_fails_too()
 
 std::weak_ptr<MyClass> weak = weak_from_this();
 std::shared_ptr<MyClass> strong = weak.lock();
 // This assert fails
 assert(strong.get());
 
;

int main()

 std::shared_ptr<MyClass> obj = std::make_shared<MyClass>();

 obj->this_fails();
 obj->this_fails_too();

share|improve this question

edited 3 hours ago

curiousguy

5,02922 gold badges3030 silver badges4646 bronze badges

asked 8 hours ago

LubosDLubosD

32311 silver badge1212 bronze badges

share|improve this question

edited 3 hours ago

curiousguy

5,02922 gold badges3030 silver badges4646 bronze badges

asked 8 hours ago

LubosDLubosD

32311 silver badge1212 bronze badges

edited 3 hours ago

curiousguy

5,02922 gold badges3030 silver badges4646 bronze badges

edited 3 hours ago

curiousguy

5,02922 gold badges3030 silver badges4646 bronze badges

asked 8 hours ago

LubosDLubosD

32311 silver badge1212 bronze badges

asked 8 hours ago

LubosDLubosD

32311 silver badge1212 bronze badges

2 Answers
2

active

oldest

votes

11

You must inherit publicly from std::enable_shared_from_this. Inheriting privately doesn't help - std::shared_ptr can't access the base class and set it up properly.

share|improve this answer

answered 8 hours ago

Igor TandetnikIgor Tandetnik

35.4k33 gold badges3838 silver badges6161 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  That must be the one of the most counter-intuitive behaviors I have seen. Why doesn't it cause compile time errors? -- Explanation is here: stackoverflow.com/questions/39937112/…
  
  – LubosD
  8 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @LubosD I don't see why you would expect a compiler error. From the compilers point of view, inheriting privately is fine and calling shared_from_this is fine. That it doesn't give the desired run-time behaviour is not the compilers problem. The code is well formed C++ regardless of whether it gives you the expected behaviour or not.
  
  – Jesper Juhl
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @LubosD There is no easy way to give a compile error. The std::shared_ptr constructor has no mechanism to tell whether you made a mistake by privately inheriting from std::enable_shared_from_this or whether you just privately inherited from some class that publicly inherited from std::enable_shared_from_this. The shared_from_this function has no way of knowing the most derived class type so it also cannot detect the programmer's error.
  
  – Brian
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well, the standard could mandate checking for base with std::is_base_of, and then complaining if it is ambiguous or inaccessible.
  
  – Deduplicator
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @LubosD What's counter intuitive is that a class isn't required to derive from std::enable_shared_from_this but something will occur only if it does. This is very dirty and will not work if you allocate another class that uses membership instead of inheritance. Just have your own weak_ptr and maintain it yourself.
  
  – curiousguy
  2 hours ago
  

  
  
  
  

add a comment | 

10

You have to publicly inherit from std::enable_shared_from_this in order for it to work.

share|improve this answer

answered 8 hours ago

NathanOliverNathanOliver

108k1919 gold badges161161 silver badges240240 bronze badges

add a comment | 

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11

You must inherit publicly from std::enable_shared_from_this. Inheriting privately doesn't help - std::shared_ptr can't access the base class and set it up properly.

share|improve this answer

answered 8 hours ago

Igor TandetnikIgor Tandetnik

35.4k33 gold badges3838 silver badges6161 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  That must be the one of the most counter-intuitive behaviors I have seen. Why doesn't it cause compile time errors? -- Explanation is here: stackoverflow.com/questions/39937112/…
  
  – LubosD
  8 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @LubosD I don't see why you would expect a compiler error. From the compilers point of view, inheriting privately is fine and calling shared_from_this is fine. That it doesn't give the desired run-time behaviour is not the compilers problem. The code is well formed C++ regardless of whether it gives you the expected behaviour or not.
  
  – Jesper Juhl
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @LubosD There is no easy way to give a compile error. The std::shared_ptr constructor has no mechanism to tell whether you made a mistake by privately inheriting from std::enable_shared_from_this or whether you just privately inherited from some class that publicly inherited from std::enable_shared_from_this. The shared_from_this function has no way of knowing the most derived class type so it also cannot detect the programmer's error.
  
  – Brian
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well, the standard could mandate checking for base with std::is_base_of, and then complaining if it is ambiguous or inaccessible.
  
  – Deduplicator
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @LubosD What's counter intuitive is that a class isn't required to derive from std::enable_shared_from_this but something will occur only if it does. This is very dirty and will not work if you allocate another class that uses membership instead of inheritance. Just have your own weak_ptr and maintain it yourself.
  
  – curiousguy
  2 hours ago
  

  
  
  
  

add a comment | 

11

You must inherit publicly from std::enable_shared_from_this. Inheriting privately doesn't help - std::shared_ptr can't access the base class and set it up properly.

share|improve this answer

answered 8 hours ago

Igor TandetnikIgor Tandetnik

35.4k33 gold badges3838 silver badges6161 bronze badges

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  That must be the one of the most counter-intuitive behaviors I have seen. Why doesn't it cause compile time errors? -- Explanation is here: stackoverflow.com/questions/39937112/…
  
  – LubosD
  8 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @LubosD I don't see why you would expect a compiler error. From the compilers point of view, inheriting privately is fine and calling shared_from_this is fine. That it doesn't give the desired run-time behaviour is not the compilers problem. The code is well formed C++ regardless of whether it gives you the expected behaviour or not.
  
  – Jesper Juhl
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @LubosD There is no easy way to give a compile error. The std::shared_ptr constructor has no mechanism to tell whether you made a mistake by privately inheriting from std::enable_shared_from_this or whether you just privately inherited from some class that publicly inherited from std::enable_shared_from_this. The shared_from_this function has no way of knowing the most derived class type so it also cannot detect the programmer's error.
  
  – Brian
  8 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Well, the standard could mandate checking for base with std::is_base_of, and then complaining if it is ambiguous or inaccessible.
  
  – Deduplicator
  7 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @LubosD What's counter intuitive is that a class isn't required to derive from std::enable_shared_from_this but something will occur only if it does. This is very dirty and will not work if you allocate another class that uses membership instead of inheritance. Just have your own weak_ptr and maintain it yourself.
  
  – curiousguy
  2 hours ago
  

  
  
  
  

add a comment | 

You must inherit publicly from std::enable_shared_from_this. Inheriting privately doesn't help - std::shared_ptr can't access the base class and set it up properly.

share|improve this answer

answered 8 hours ago

Igor TandetnikIgor Tandetnik

35.4k33 gold badges3838 silver badges6161 bronze badges

share|improve this answer

answered 8 hours ago

Igor TandetnikIgor Tandetnik

35.4k33 gold badges3838 silver badges6161 bronze badges

answered 8 hours ago

Igor TandetnikIgor Tandetnik

35.4k33 gold badges3838 silver badges6161 bronze badges

answered 8 hours ago

Igor TandetnikIgor Tandetnik

35.4k33 gold badges3838 silver badges6161 bronze badges

10

You have to publicly inherit from std::enable_shared_from_this in order for it to work.

share|improve this answer

answered 8 hours ago

NathanOliverNathanOliver

108k1919 gold badges161161 silver badges240240 bronze badges

add a comment | 

10

You have to publicly inherit from std::enable_shared_from_this in order for it to work.

share|improve this answer

answered 8 hours ago

NathanOliverNathanOliver

108k1919 gold badges161161 silver badges240240 bronze badges

add a comment | 

You have to publicly inherit from std::enable_shared_from_this in order for it to work.

share|improve this answer

answered 8 hours ago

NathanOliverNathanOliver

108k1919 gold badges161161 silver badges240240 bronze badges

share|improve this answer

answered 8 hours ago

NathanOliverNathanOliver

108k1919 gold badges161161 silver badges240240 bronze badges

answered 8 hours ago

NathanOliverNathanOliver

108k1919 gold badges161161 silver badges240240 bronze badges

answered 8 hours ago

NathanOliverNathanOliver

108k1919 gold badges161161 silver badges240240 bronze badges